Adoption law has been labelled outdated and discriminatory by an independent tribunal.
In a newly-released decision, the Human Rights Review Tribunal said the 61-year-old Adoption Act discriminated against people based on sex, marital status, disability and age.
It said the law breached the Human Rights Act and the Bill of Rights Act.
The act places restrictions for adopting on single men trying to adopt a female child, civil union partners, same-sex de facto couples and anyone under the age of 25.
It also prevents any adoptee under 20 from obtaining a copy of their original birth certificate.
Reform group Adoption Action, which brought the case against the Attorney-General, said the law was out of touch with social attitudes and values.
The group's spokesperson, Anne Else, urged the government to act.
"The problem with this decision is that it comes on the back of years of the government being told very firmly, and both Labour and National governments, that the act is extremely out-of-date," she said.
"The United Nations has also told us the same thing in no uncertain terms."
Justice Minister Amy Adams was yet to comment, but told RNZ News late last year adoption law reform was not on her agenda.
Years of criticism
The Law Commission, the Law Society, the Children's Commissioner and UNICEF have all said the act is out of date.
The Law Commission published a report in 2000 suggesting about 100 changes to the law but, to date, none had been imposed.
In 2007, a Ministry of Justice paper to Cabinet stated there were legal and social reasons why adoption laws needed to be changed because it perpetuated discriminatory practice and created a system open to abuse.
Three years later, a Human Rights Commission report highlighted the need to review adoption legislation as too many children were experiencing poverty and poor health, which resulted in poor outcomes in early childhood, which reached into later childhood and adulthood.
Labour Party justice spokesperson Jacinda Ardern said reform should be a priority.
"It's no wonder that adoption numbers are so low in New Zealand. This law was passed in the 1950s and was based around the idea of stranger adoption - it doesn't even cater for whangai adoption despite it being common practice in New Zealand," she said.
"The minister and government has 120 days to respond to the Human Rights Tribunal. We urge her to use this opportunity to prioritise adoption reform - the time has most certainly come."
A bill sponsored by Ms Ardern was shot down in Parliament in 2013, and she said she could not explain why there had been opposition to reform.
"You can speculate but really only the government could tell you. We've had successive ministers ... who have acknowledged that reform is needed but it simply just hasn't been a high priority for the government, and that was certainly the response I've received from successive ministers of justice," she said.